The production of slivers and webs which are as regular as possible in cross section is very significant in the textile industry. Slivers and webs of this type are mostly produced on cards or on carding engines. A large number of methods and apparatus for automatically controlling the fiber supply to these machines is known for achieving the required regularity.
A known method, such as that disclosed in U.S. Pat. No. 3,435,673, comprises measuring the cross section of the issuing slivers and controlling the speed of the feed roller of the card based on the measured values obtained, such that even when the density of the fiber feed varies, just enough material is always supplied, per unit of time, to the card as is required for maintaining a constant cross section in the material drawn off. This method is relatively slow due to the long fiber transport path between the positioning point (feed roller) and the measuring point at the draw-off rollers. Fluctuations which are shorter than this transport path cannot be regulated, because they have already passed the positioning point at the time of their metrological detection. It is only possible to accelerate this method by bringing the measuring point closer to the positioning point. However, an absolute measuring member is not known from the present prior art which may be mounted downstream of the feed roller in an optimum vicinity and which simultaneously has the precision required for the purpose of use.
Another known method, such as that disclosed in U.S. Pat. No. 4,271,565, solves this problem in that in addition to positioning the absolute measuring member at the draw-off rollers, another measuring member is positioned at the lickerin, or at the cylinder, or at the doffer, or at the doffer rollers, or at the web, which must only recognize variations in the fiber throughput, but not the absolute amount thereof. Measuring members of this type may also be produced from the present prior art. In the combination mentioned with a precise absolute measuring member, they allow the desired acceleration in the control procedure even with a relatively low precision.
A known measuring member belonging to this category measures variations in the fiber throughput by an optical determination of the density of the fiber clothing, for example on the cylinder. The method is based on the fact that, for example, a lightly colored fiber clothing reflects more light with an increasing density than the dark background formed, for example, by the cylinder clothing. This measuring member does indeed constitute substantial technical progress. However, the necessary difference in the reflection characteristics of the fiber clothing and the background implies a restriction in the practicability. A further restriction is the decreasing precision where there is a poor fiber resolution. For example, this obstructs optimum use of this measuring member at the licker-in although the licker-in would be the optimum measuring point from the control point of view, for the card feed control with a closed-control loop.